ICCC'5 Internatonal Carpathan Control Conference ICCC 5 Mskolc-Lllafüred, Hungary May 4-7, 5 FPGA Implementaton of the Reconfgurable Control System for AC Drves J. VÁSÁRHELYI, Cs. SZABÓ, I. I. INCZE, T. M. IMECS ÁDÁM Department of Automaton, Unversty of Mskolc, Mskolc-Egyetemváros, Mskolc, Hungary Phone: +36 46 565, Fax: +36 46 438; E-Mal: {vajo, adam}@mazsola.t.un-mskolc.hu Department of Electrcal Drves and Robots, Techncal Unversty of Cluj-Napoca, P.O., Box 99, Cluj-Napoca, Romana, Phone: +4-64-4-4, Fax: +4 64 5955; E-Mal: emal: {mecs, csaba.szabo, oan.ncze}@edr.utcluj.ro Abstract - The paper focuses on the mplementaton n Feld Programmable Gate Arrays (FPGA) of vector control systems of the nducton motor suppled from the tandem (hybrd) statc-frequency converter. Reconfgurable control structure ensures dfferent strateges for operatng modes wth non-faled and partal-faled converter. The reconfguraton process ntroduces perturbatons n the vector controlled AC drves. The paper tres to gve a synthess of the tandem vector control structure mplementaton. Problems related to hardware mplementaton of the transton from a control structure to another are dscussed. Keywords: FPGA, embedded system, vector control, AC drves.. Introducton In 984 a new technology the Feld-Programmable Gate Array (FPGA) for mplementng dgtal logc was ntroduced. The applcatons, whch frst were manly n mplementng optmzed two-level logc applcatons, today has developed to home networkng, telecommuncatons, dgtal sgnal processng, and real-tme control applcatons. Many of the FPGAs were confgured by statc RAM cells, whle other FPGAs were confgured usng antfuse technology. Today the volatlty has become a need n some applcatons and programmng s no more consdered a lablty.
The new technology and the programmng methods of the FPGA chps, whch are named reconfguraton of the FPGA, and has generated a new research feld named reconfgurable computng. Most of applcatons of reconfgurable computng were reported n mage processng, dgtal sgnal processng and custom computng machnes. Wth the evoluton of FPGA technology and the assocated desgn tools, the applcaton feld has grown rapdly. Even more, ths technology became a support for the new sngle chp soluton called System on Chp (SOC). In the md 99s the evoluton of FPGAs allowed the mplementaton of smple dgtal sgnal processng algorthms, and as a result of ths possblty they have been ntroduced n motor control feld. Up to now Crstea, Imecs, Monmasson, Poure and ther research groups reported applcaton of reconfgurable hardware n the mplementaton of motor control system for AC Drves [,, 9 and ]. Consderng that the man advantages of FPGAs regardng ther reach-hardware resources and parallel algorthm mplementaton possbltes, but also n the run-tme hardware-structure reconfguraton possblty, we may say t wll be the bass of many future vector control applcatons.. Reconfguraton of the Control System Vector Control Systems for AC drves are charactersed by hgh dynamc changes, and yet t s technologcally mpossble to comple the next confguraton n run-tme for such a system. The reconfguraton of vector control systems s applcable only f the next confguraton s known at comple tme, whle the duraton of the actual confguraton s known or may be unpredctable. Ths mples two types of reconfguraton: The reconfguraton s used to mprove the workng condtons mposed by the speed, torque, or other varable. The reconfguraton s used to mantan the fal-safe operaton of the drve. For both cases, we can assgn a dscrete state machne, whch s equvalent to a logc sate machne [6]. The condtons, whch determne the start of the reconfguraton, are varous and depend on the applcaton on hand. The necessty of reconfguraton n motor control s based upon the practcal observatons that the performances of varous types of vector-controlled drves are dfferent. These dfferences depend on the range of speed, mechancal load characterstcs, sensors, and the type of the supply power electronc converter. The reconfguraton for nducton motor drve control was ntroduced to mprove the control system characterstc and later on for the fal-safe operaton of the tandem converter. These two mentoned applcatons of the reconfgurable computng n nducton motor control mply two reconfguraton cases: Case I: the motor drve s suppled by only one nverter and the reconfguraton s appled to mprove the workng condtons of the motor. Case II: the motor drve s suppled by two nverters -.e. the tandem converter and the reconfguraton of the control system s used for fal-safe operaton of the drve n the case f one of the nverters fals.
In both case the control of the executon element, represented by the AC motor together wth the PWM and the electrcal and mechancal sensors assembly, mpose real-tme performance of the control system algorthm. One may conclude that decreasng the samplng perod the control system can perform better. On the other hand decreasng the samplng perod can mpose very short reconfguraton tme (partal or total). Both cases were brefly treated for vector control systems n [3] The challenge for an embedded system engneer s represented by case I. In ths case t s also dffcult to fnd an ndustral applcaton. In reconfguraton case II we found a possble applcaton, whch s charactersed by the so called tandem nverter [3]. An alternatve soluton for medum- and hgh-power AC drves s the tandem statc frequency converter (SFC) fed nducton motor. Ths confguraton s a hybrd SFC, whch combnes the advantages of two, parallel workng, dfferent types and dfferent power ranges DC-lnk converters. A large power Current Source Inverter (CSI), operatng n Pulse Ampltude Modulaton (PAM) converts the actve power, and a small power Voltage Source Inverter (VSI) workng n Pulse Wdth Modulaton (PWM) and supples the reactve power requred for mprovng the qualty of the motor currents. To obtan the best dynamc behavour the control of the tandem-converter-fed nducton motor can be acheved usng conventonal vector-control structures [8]. Let suppose one of component converter (VSI or CSI) fals. Ths means, the control structure loses ts tandem character and the control structure wll work wth only one nverter. Under these new workng condtons, the control structure prevously appled to the tandem converter s no more effcent. To keep the drve workng the control structure need to be reconfgured correspondng to the workng nverter character. Due to the reconfguraton, the vector control system s mplemented n Feld Programmable Gate Array. Unlke other vector control system mplementatons [, 9 and ] the mplementaton presented n ths paper explots all the parallelsm of the vector control algorthm. For ths reason the hardware resources consumed by the mplementaton are very hgh. The advantage of parallel mplementaton s the hgh performance of the control system and the possblty to obtan very short samplng perods. The reconfguraton of a vector control system can be motvated by the fal-safe operaton of a plant. The plant (.e. the control system, the nverter(s), the motor and the sensors) has to be kept on workng n order to fnsh ts msson. In ths case, the reconfguraton moment t r s not known and the next confguraton s known. Ths reconfguraton case s appled to the tandem converter f one of the nverters (VSI or CSI) fals. Ths means, that the control structure loose ts tandem character and the control structure wll work wth only one nverter. Under these new workng condtons, the control structure prevously appled to the tandem converter s no more vald. To keep the motor workng the control structure need to be reconfgured correspondng to the workng nverter character. The power-on ntalzaton.e. startng the drve, t s realzed, that ether startng the control system wth the tandem converter, or ether startng t wth CSI converter structure. Ths resulted from the obtaned smulaton results. Up to now the state transton graph was presented n [6, 7] wth the power-on nt n STATE. New results proved that the start and power-on-nt s mposed by the value of the startng currents, whch can be very hgh and depends on the control structure. For ths reason n some crcumstances the start should be made wth the CSI/VSI (STATE or STATE3) nverter and after the system acheves ts nomnal workng condtons t should be reconfgured for the tandem converter structure.
Power-on Int STATE TANDEM CONVERTER CSI fals or nomnal condton CSI OK or nomnal condton VSI fals VSI OK or nomnal condton STATE3 VSI INVERTER Power-on Int Power-on Int STATE CSI INVERTER Fg.. The state transton graph of the tandem converter system The tandem converter needs dfferent control strateges dependng on the type of the PWM procedure used for the VSI. The selected PWM procedure can change the source character of the VSI and of the tandem converter, too [Hba! A hvatkozás forrás nem található.]. The open-loop voltagecontrol PWM procedures,.e. carrer wave or Space-Vector Modulaton (SVM), keep the voltage-source character of the VSI, but usng closed-loop current-control PWM procedures (e.g. the common bang-bang current control) the behavor of the VSI becomes of current-source character. The tandem converter s sensble to the VSI falure. If the VSI fals then the control system structure loose ts voltage character and needs to be reconfgured n order to keep on moton the drve and to adapt the control system structure correspondng to the new demands. The CSI mpose the new control structure character.e. a current one.the three fundamental crtera, determnng the structure of a vector control system.e. the orentaton-flux, the feld dentfcaton method, and the type of the used PWM procedure appled to command the converter, nfluence the control system structure of the tandem converter. For ths reason, from the ntally proposed tandem converter structure gven by Trzynadlowsk, Imecs et all. proposed dfferent tandem structures. The transformaton of these structures n reconfgurable ones resulted n completely new control system structures for AC drves [3, 5, ]. The orentaton flux (rotor - Ψ r or stator - Ψ s ) and the PWM method appled to the converter (.e. space vector modulaton or current feedback modulaton) gve threereconfguraton possbltes as shown below n Table. There are two types of possble reconfguraton models -.e. partal or total reconfguraton. The method s used n the mplementatons s a total reconfguraton for the CSoC and partal reconfguraton for the FPGA hardware support. For technologcal reasons, durng the research process, smulaton and mplementaton the total reconfguraton model was chosen, whch reconfgures the control system as a whole. Table. Reconfguraton possbltes of the tandem converter [3] CSI-fed motor Tandem converter/vsi-fed motor Descrpton State (State ) n references Orent Orentato aton n Flux Flux PWM Method Ψr Ψr SVM [3,] Ψr Ψs SVM [5, ] Ψr Ψr Current Feedback Modulaton ( bang-bang ) [4]
The basc dea s straghtforward: After power-up the control system of the AC drve works as tandem converter (STATE of the state machne), when the VSI fals wll be reconfgured to work as CSI converter (STATE of the state machne). A possble representaton of the transton from one state to the other, n fact, may be a demultplexer and a multplexer. However, one should note that, whle these components may be ndeed possble mplementatons, they are ntended to be abstract enttes dd not need any mplementaton. 3. Current Controlled PWM-VSI-fed Inducton Motor wth Rotor-Feld Orentaton Due to the voltage-source character of the tandem converter, the motor absorbs freely ts stator currents. Consequently, the VSI wll be the actuator ensurng the vector control of the nducton motor drve. It s possble to apply the common PWM procedures (voltage- or current controlled ones) characterstc to the VSI. Applyng to the VSI current-controlled-pwm, n manner of the bang-bang converter, the tandem-converter-fed motor wll be controlled n fact n current. Constant swtchng frequency s obtaned usng synchronzed on-off swtchng controllers. The abovementoned procedures are approprate for feld-orentaton-based tandem-fed drves. In Fg. the nducton motor operates suppled from the both converters n tandem mode (correspondng to poston of the multplexers). Supposng a falure of the VSI the control structure has to be adapted to the new workng condton,.e. runnng suppled only by the CSI (poston of the multplexers). The reconfguraton of the hardware structure realzes the vector control system needed for the workng nverters. Due to ts smplcty, both operatng modes use rotor-feld orentaton. In the tandem-fed mode, the VSI operates wth current feedback loops. Because of the dffcultes encountered by drect measurement of the modulated-voltage waves, the stator voltage s dentfed n block V s Id usng the measured DC-lnk voltage and the state of nverter swtches accordng to the PWM logc takng nto account the voltage losses on semconductor devces, too. Based on the stator-voltage and current components transformed n d-q reference frame, the block Ψ s C ntegrates the natural stator-voltage equatons yeldng at ts outputs the statorflux d-q components. In order to obtan the orentaton flux, the block Ψ r Co compensates the stator flux. The vector-analyzer VA computes the ampltude and the angular poston of the orentaton feld. The reference values of the stator-current space-phasor components are obtaned from the flux- and speed-control loops. After the coordnate transformaton, they wll be transformed to the three-phase references of the hysteress dead band current-controllers [3, ].
ω Ψ r r + - + - Ψr Flux Controller Speed Controller Identfed Feld Coordnate Transformaton sd λ r sd λ r cosλr CooT [D(-λr)] VA snλr ψrd ψrq sd sq ψrco sd sq ψsd ψsq sd sq CSI Current s ψ sc sd sq vsd vsq π 3 VA PhT [A] - PhT [A] PhT [A] DC + Phase Transformaton εs DC-lnk Current Controller α - DC Synchronsaton [s ] + - PWM logc [vs] [s] fs εcsi V sid Thyrstor Rectfer PAM-CSI [vs]* AC lne Ld Dode Rectfer 3xC Cd PWM- VSI ωr Mechancal Angular Speed Rotor-flux Compensaton Stator-flux Computaton Fg.. Reconfgurable Rotor-feld Orented Vector Control System wth Current Feedback Modulaton for the Tandem Converter-Fed Inducton Motor [4]. 4. Algorthm Analyses and Modularty The creaton of a module lbrary was motvated by the fact that the smulaton of the reconfguraton process s not possble or t s dffcult whle no tools exst for ths knd of smulaton. On the other hand, recently t has become possble to mplement dgtal sgnal processng algorthms on FPGAs drectly from Matlab Smulnk envronment. The elements of the lbrary are the most common modules of vector control systems, and each present a standalone unt n the lbrary. As a result of ths ndependency, the vector control system can be syntheszed by module or as a whole. The modularty s mportant when the mplementaton target s reconfgurable hardware such as FPGAs []. Analyzng the work of parallel research groups Crstea [], Monmasson [], Pourre [9] t can be concluded that those mplementatons were orented for specal purposes and they used confgurable logc, but dd not study the reconfguraton as a possblty for vector control systems. The equatons of vector control schemes can be decomposed n elementary mathematcal operatons. What s more, these elementary operatons can be combned n the most used DSP functon multply and accumulate : c = k = ( a b ) () The dfference between the DSP and FPGA mplementaton of the MAC s that n the case of latter one the operatons from () are executed n parallel and not sequentally. In such a way, the executon tme s reduced by the parallel computaton. The parallel mplementaton of the algorthm results n very fast executon speed. For ths reason, the samplng perod can be decreased untl the technology and the PID controllers allow t. The parallel computaton of equatons gves a sgnfcant mprovement compared to the DSP sequental computaton. The parallel mplementaton method dsadvantages could be the
ntensve hardware resource consumng and the prce pad for a chp. Another advantage of the parallel mplementaton s that partal reconfguraton became possble for each computaton module. Ths allows only the parts of the control system to be reconfgured, whch are not common for two control structures. We can conclude that from the pont of vew of the reconfguraton, ndependent from whch type of reconfguraton method s used, the parallel mplementaton s the optmum one. The sequental mplementaton methods do not allow very short samplng perods comparng to the parallel one. The executon speed of the MAC functons has to be fast enough, n order to compute all the equatons of the control system n only one samplng perod tme. From the vew pont of reconfguraton the sequental mplementaton method s not an optmal soluton. Ths s explaned by the fact that the reconfguratons of the vector control system nvolve changes n the output varable characterstcs (voltage/current) and modfy the type and value of varables and constants when the new MAC functons are executed. From the above reasonng results that the parallel mplementaton was chosen as a soluton for the mplementaton of the IP lbrary and the reconfgurable vector control system. Some functons of the vector control system cannot be mplemented wth the above-mentoned MAC functon, n these cases partcular solutons are appled. In [3] there are shown the mplementaton results of the module lbrary and the estmated hardware resources needed for the mplementaton of the control system shown n Fg.. 5. Smulaton Results In all the three possble reconfguraton varants presented n Table, the smulatons were performed wth smlar condtons n MATLAB-Smulnk envronment. The motor data are: 5.5 kw, 5 Hz, Vrms, 4 Arms and 4 pole-pars. The motor was started controlled by the tandem nverter and after some tme (usually.5s or s after start) the control structure was reconfgured due to the faled VSI. The smulatons were concentrated on the moment of the reconfguraton and the effects on the motor varables f the control structure s reconfgured. The research was not extended to the fault detecton, whch s not the subject of ths research. We presumed that the moment when the VSI fal could be detected n a way (for example detectng the equalty of three voltages u sa =u sb =u sc = at the same tme). The smulaton results presented n ths paper were made for the control scheme presented n Fg.. The smulaton results show that the transents ntroduced by the reconfguraton nfluences all the varables as presented n []. Fg. 3. Current waveforms before and after reconfguraton. Fg. 4. Rotor and Stator resultant flux before and after reconfguraton.
Fg. 5. Electromagnetc torque, Electrc angular speed. Fg. 6. Mechancal characterstcs of the nducton machne Fg. 7. Stator current on phase "a". Motor Stator-current tme-space-phasor dagram. Fg. 8. VSI/Capactor output currents. CSI output current tme-space-phasor dagram. 6 5 4 sd [A] CSId [A] - -4-6 6 4 - [A] sq -4-6.5 tme[s].5-5 6 4 - [A] CSIq -4-6.5 tme[s].5 Fg. 9. Motor Stator-current space-phasor dagrams. VSI or Capactor output-current tme-space-phasor dagram. Fg.. CSI output current space-phasor dagrams. Tme -space-phasor dagram of the controlled/resultant rotor-flux. 3.5 VSID / Cd [A] - ps rd [Wb] -.5 - -3 4 VSIq / Cq [A] - -4 Fg.. VSI or Capactor output-current space-phasor dagrams..5 tme[s].5 -.5 ps rq [Wb] -.5 - Fg.. Space-phasor dagrams of the controlled/resultant rotor-flux..5 tme[s].5
From the smulaton results one can conclude that the transton from one control structure to the other generate transents n the motor. A transent management soluton was proposed n [5] 6. Conclusons The reconfgurable vector control system for tandem nverter s a new soluton for AC drves. It mproves the workng condtons of the drve, also assures fal safe operaton. Vector control systems for AC drves are characterzed by hgh dynamc performance. For the reconfguraton of such a system yet t s technologcally mpossble to comple the next confguraton n run-tme. The reconfguraton s applcable only f the next confguraton s known at comple tme, whle the duraton of the actual confguraton s unknown or can not be predcted. Snce the smulaton of the reconfguraton process n MATLAB Smulnk envronment was modelled by multplexers. In the mplementaton these multplexers were mplemented. In such a way a reconfgurable control algorthm was proposed. The research concentrated on the fal of the VSI. If the CSI fal the vector control system together wth the VSI and the motor stll can work, but the performances wll be lower then the tandem nverter performances. For mplementaton of reconfgurable vector control structures CSOC and FPGA chps are recommended. The created Module Lbrary, lke other Matlab tools, helps the rapd prototypng and mplementaton of vector control systems for AC drves targetng FPGA chps. The module parameters are freely modfable on demand. The Module Lbrary allows the smulaton of the reconfguraton process and the study of the reconfguraton effects upon the behavours of the AC drves. Comparng wth other exstng FPGA mplementatons the mplementaton results presented n ths paper allows the parallel mplementaton of any vector control system, n contrast wth other mplementatons based on sequental computaton. The advantage of parallel mplementaton s the ncreased computaton speed and low samplng perod. The dsadvantage may be consdered the hgh number of FPGA cell consumpton, whch mples the use of rch resource FPGA chps. Acknowledgment The Tandem Inverter was subject of a research project carred out at the Insttute of Energy Technology, Aalborg Unversty, Denmark. Thanks to Prof. A. Trzynadlowsk from Nevada Unversty, Reno, USA for the collaboraton and to Prof. F. Blaabjerg from the Aalborg Unversty and to the Danfoss Drve A/S, Denmark for ther generous support. The authors are grateful to Xlnx Inc. for donatons. Ths publcaton s subject of the scentfc and technologcal Hungaran-Romanan ntergovernmental and sponsored by the Department of Development and Research of the Hungaran Mnstry of Educaton and ts contract partner the Romanan Mnstry of Educaton Research and Youth, research s part of the Project TET 6/3.
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